Muscle strips from newborn and adult rabbits were stimulated with acetylcholine (ACh; receptor activation), potassium (membrane depolarization), and calcium (skinned muscle preparation). The study was designed to determine whether postnatal development is associated with agonist-specific increases in force development or with an overall increase in antral smooth muscle contractility. The results can be summarized as follows. 1) Antral smooth muscle from newborn animals developed less active force than tissues from adult animals when stimulated with ACh. 2) Antral smooth muscle from newborn animals developed less active force than tissues from adult animals when stimulated with potassium. 3) Skinned muscle preparations from newborn animals developed less active force than skinned muscle fibers from adult animals when stimulated with calcium. The data suggest that the overall contractility of antral smooth muscle from the newborn is less than that of the adult and that the difference can be attributed, at least in part, to factors past the level of the membrane. The study does not rule out the possibility that differences might also exist with respect to the availability or mobilization of calcium.